1. Field of the Invention
The present invention relates to radio communication terminals, and in particular, to a power saving device for reducing current consumption during a telephone conversation.
2. Description of the Related Art
In a CDMA (Code Division Multiple Access) or PCS (Personal Communication System) terminal, an RF (Radio Frequency) power amplifier is so designed as to have a maximum output power of +28dBm in order to satisfy the IS-98 standard and the J-STD-008 standard. In addition, a transmitter of the CDMA or PCS terminal includes an automatic gain control (AGC) amplifier for controlling the power level according to the strength of a received signal or a power control command transmitted from a base station. Accordingly, the terminal should increase the transmission power in an area where a signal transmitted from the base station has a low signal strength and decrease the transmission power in an area where the signal transmitted from the base station has a high signal strength.
In the meantime, the CDMA or PCS terminal, employing QPSK (Quadrature Phase Shift Keying) modulation, requires a linear amplifier. Therefore, the AGC amplifier adopts an A-class or AB-class bias. In such a case, however, the terminal consumes a constant current regardless of the transmission power.
FIG. 1 illustrates a conventional two-stage power amplifier. As illustrated, a driver amplifier 101 primarily amplifies an input RF signal RFI, and a final amplifier 103 secondarily amplifies the amplified RF signal output from the driver amplifier 101. The driver amplifier 101 and the final amplifier 103 are both provided with a fixed level control signal (or a fixed gain control signal) to control the power level. Therefore, the amplifiers 101 and 103 consume a constant idle current regardless of the strength of the signal. This results in an increase in the current consumption.
To solve this problem, there is proposed a method of providing a variable voltage to the power amplifier to control a linear region of the power amplifier, thereby reducing the current consumption.
FIG. 2 illustrates an improved two-stage power amplifier. In the drawing, a variable voltage is provided to the driver amplifier 101 and the final amplifier 103. When the transmission power is low, the variable voltage is lowered to reduce the idle current to the minimum value. In this way, it is possible to reduce the current consumption to some extent. However, each of the elements constituting the power amplifier have a different linear region. When some of the elements have a narrow linear region, it is difficult to obtain the desirable power saving effect. Furthermore, no matter how little the idle current flow may be reduced to, each stage of the power amplifier consumes the constant current. Thus, there is a limitation in the ability to reduce the current consumption. That is, even when the transmission power is low enough such that it is not necessary to drive the final amplifier, the idle current continues to flow into the final amplifier, wasting the current.
It is therefore an object of the present invention to provide a power saving device for reducing current consumption by changing the number of power amplifying stages in a radio communication terminal.
To achieve the above object, there is provided a power saving device for a radio communication terminal including a first amplifier for high power and a second amplifier for low power including a signal path selector for switching an input signal to an input node of the first or second amplifier in response to a switching control signal. An output path selector selects one of the output signals from the first and second amplifiers in response to the switching control signal. An RSSI detector detects the signal strength of a signal received from a base station. A controller analyzes the detected signal strength, and generates a switching control signal of a first state, when the signal strength is low, to enable the signal path selector to switch the input signal to the first amplifier and enable the output path selector to select the output signal of the first amplifier. The controller also generates a switching control signal of a second state, when the signal strength is high, to enable the signal path selector to switch the input signal to the second amplifier and enable the output path selector to select the output signal of the second amplifier. A power supply blocks a first supply voltage to the first amplifier in response to the switching control signal of the second state, and blocks a second supply voltage to the second amplifier in response to the switching control signal of the first state.